VoLTE/VoWiFi is an IP-based voice transmission technology in which all services are carried on a 4G/WiFi network, and can implement transmission of data and voice services on a same network. SIP (Session Initiation Protocol, Session Initiation Protocol) signaling is used for voice communication. A standard VoLTE or VoWiFi call procedure is shown in FIG. 1. A mobile terminal user Alice intends to call a mobile terminal user Bob. A mobile terminal of Alice sends an INVITE message to a server. The server forwards INVITE to a mobile terminal of Bob. After Bob answers the call, the mobile terminal of Bob returns a 200 OK message to the server. The server returns the 200 OK message to the mobile terminal of Alice. The mobile terminal of Alice returns an ACK (Acknowledgement, acknowledgement character) to the server. The server returns the ACK to the mobile terminal of Bob. In this way, the call is set up between Alice and Bob. When Alice intends to request to hold the call, a procedure is shown in FIG. 2. The mobile terminal of Alice sends an INVITE (hold) message to the server. The server forwards INVITE (hold) to the mobile terminal of Bob. The mobile terminal of Bob returns a 200 OK message to the server. The server returns the 200 OK message to the mobile terminal of Alice. The mobile terminal of Alice returns an ACK to the server. The server returns the ACK to the mobile terminal of Bob. In this way, a call hold operation is completed between Alice and Bob.